if the image uploader fails to validate your file, then it is probably because Magento is unable get Mime Content Type.
The main reason for this error is: function mime_content_type does not exist (check your php version to find out why)
Solution: Open the file vendor/magento/module-cms/Model/Wysiwyg/Images/Storage.php (tested and it worked with Magento 2.3.0)
and replace it by the attached one.